Just been looking at the price of polyurethane sheets and decided they could definately be cheaper at £38 for 2400mm x 1200mm x 50mm (could get two boards outta that). Anyway you can buy expanding PU foam which expands something like 25x its initial volume! So at about £50 for 2kg, you could get a lot of boards for that.

First thought were to make an enclosed mould with thin plywood sheets with a release film so its all re-useable. Top sheet would have to have holes in it for excess foam to leak from. Would also eliminate a lot of shaping time. Just finishing, excess foam removal and rail shaping to do. Would generally save all the waste material you get when shaping from sheets too.

The pressure generated by the expanding PU foam would easily crack up a plywood mold.
This is something boat builders have to be careful about when using this for floatation....
Plus, once cured this type of foam is too brittle, and it would fail in shear in a kiteboard....
Unfortunately, it's not really the same as a PU surfboard blank.
To get that, or a strong molded pop-out, you need a factory.

If you're using spray foam that's housing related, you might try to find a low expansion/pressure foam. Big mistake people make is they use the regular cans of foam to install windows and it expands to forcefully, it'll pop windows or warp things. This might solve your mold issue.

Perhaps you can make a mold of the top and bottom, foam them, them glue together?

I guess you need to find a PU foam recipe to give 5-10lb/CFT density.
Ideally it would not require a massively reinforced ($$$) mold. This might be hard.
The hint about Clark using concrete molds is interesting.
Even a few psi could blow apart a mold that is small, light and cheap to build.
To get strong PU foam, you might be dealing with more than only a few psi, but it would take trial and error to find out what you needed to get the physical properties right.
Maybe concrete could work?
For example, a rough idea for a twin tip 135x40 (~54"x16"):
This would be ~800 in^2 area each side.
If the pressure was 10psi, I think you would need the mold to take 4 tons pressure.
So a concrete mold with a lid made of 3 yard of concrete (8000-10000#) would hold this pressure down. Concrete compressive strength would let the bottom half of the mold be thinner, naturally, but you would need a strong floor! Maybe you would also need a forklift.
Probably a much lighter concrete mold with rebar and I beams or really heavy angle on the perimeter to bolt it closed would be the best solution for a cheaper pressure mold.
If you can cast concrete (and weld), it might not be hugely expensive, but I am not sure if worth the trouble unless you are going to pop out hundreds of cores from the same mold. The time to engineer it and the cost might still be $$$.

Of course, molding a surfboard would mean more area, more force to resist, etc.
A custom machined metal mold could work too and be much lighter and dissipate heat faster, but cost $$$$$$. No doubt there are lots of other factors, and maybe some mistakes in my thinking.
Maybe you will find them.

Even if you could find a way to make the type of foam you need with only 0.5 psi, it would still take a pretty strong mold. What it boils down to, IMHO, is these are production techniques, rather than custom one off techniques. But maybe you can find a new kind of of foam, special recipe or new technique to make it easier!
Or for $100, get enough corecell or D-cell etc to build a few boards.